What is the cause of my sibilance problem?

I have been fighting a sibilance problem for about a year. I thought I had it beat but it is back. I've tried cleaning and treating all connections, acoustic foam treatment at first reflection points on side and front walls and ceiling, different interconnects, speaker position, and even a different digital front-end. The problem manifests itself as extremely harsh 's' sounds in both male and female voice. I am beginning to wonder if I need to have my hearing checked - I tried headphones and hear the same problem. Well, here is my system:

Aiwa XC-37M CD-changer
MSB Link DAC III with 24/96 upsampling board
B&K Ref. 10 preamp
B&K AV6000 poweramp
Paradigm Reference Studio 60 Loudspeakers
Hsu Research VTF-2 powered subwoofer
Signet center channel
Atlantic Technology surrounds
Sennheiser HD580 Headphones
Home Grown Audio Super Silver interconnects
Monster Cable subwoofer cable
Ted's Excellent Cable speaker wire (mains)
Kimber 4TC speaker wire (surrounds)
Mapleshade Brass cones, weights, and cork/rubber feet
Michael Green 5-shelf Audio Rack.

A Television, the subwoofer, and the Audio Rack sit between the louspeakers; which are approximately 7' apart and 7' from the listening position. My room is highly irregular and ASC claims that $1200 in acoustic treatment is the answer to my problem. I'm not saying they are wrong. But, having experienced the same problem with headphones (and acoustic foam not alleviating the problem one iota) I am beginning to wonder...
Sol322 - The cones I use on the transport are the Mapleshade Triplepoints and on the DAC and preamp I use Mapleshade Surefoot cones. The Triplepoints have three small points instead of a flat surface where the components comes in contact with it. For each component the cones are arrayed in a triangular pattern with the "points" "down" (one cone along the rear and two along the front). It certainly is easy enough to try different configurations and I think I'll experiment tonight (provided I get home early enough). Incidently, the problem occurs with or without the cones installed.

Detlof - While I'm not ruling out the front-end I had a Roksan Caspian CD Player that only started exhibiting the problem after several changes to my system. There-in lies the problem. I don't know if it was something I did or if the problem was always there just hidden.

Nightdoggy - I must admit to some confusion. While I think I understand the gist of your post you lost me in the details. My system does in deed have alot of wires. But I don't think there is any possibility of simplifying without removing components. Perhaps one day I'll have the room and budget for separate HT and two-channel.

There is a possibility that the culprit is the preamp. It is the one component that I haven't experimented with. And given that the problem is experienced both with headphones and loudspeakers it seems to me that eliminates the power amp (though certainly has other failings). Another interesting thing is that the sibilance is present at all volume levels - well, I haven't actually tried all since I live in an apartment and don't want to upset the neighbors. And, the sibilance is greatly reduced to non-existent with movie playback - maybe a good experiment would be to play a movie in direct stereo mode and see if the sibilance is worse or better than Dolby Digital mode.
Gallaine, after what you say now, I doubt that the front end could be the culprit. Nightdoggy might indeed have a point, with RFI being the culprit. Then also a bad solder joint or a loose connection somewhere comes to mind, but then I think you said that you had switched and experimented with cables. Experimenting with your preamp is probably your next best bet. I wish you the best of luck. The fight against this kind of nasties can be very exasperating. Please let us know of your progress!
Check to see if the problem persists w/o DAC. If it does not then you have jitter. See Sonic Frontiers for a solution.
Sorry about the silence. I haven't had much opportunity of late to try some of the suggestions posted. My deadline at work is the end of week - hopefully I can take off this weekend.

One thing I did try was to change the "mode" from direct stereo to several others. Interestingly, the problem was significantly less in surround. I am beginning to wonder if the problem is my loudpseakers. When voice is routed to the center channel the sibilance goes away - or seems to.